Here, the term quality factor refers to the following ratio:Q=f/Δf where:                Q is the quality factor,        f is the oscillation frequency of the oscillator, and        Δf is the full width at half maximum of the line centered on the frequency f in the power spectrum of this oscillator.        
Radiofrequency oscillators have derived from spin electronics.
Spin electronics uses the spin of the electrons as an additional degree of freedom in order to generate new effects. The spin polarization of an electrical current results from the asymmetry existing between the mobility of the conduction electrons of the “spin-up” type (that is to say parallel to the local magnetization) and of the “spin-down” type (that is to say antiparallel to the local magnetization). This asymmetry leads to an asymmetry of the conductivity between the two channels, spin-up and spin-down, giving a net spin polarization of the electrical current.
This spin polarization of the current gives rise to magnetoresistive phenomena in magnetic multilayers such as giant magnetoresistance (Baibich, M., Broto, J. M., Fert, A., Nguyen Van Dau, F., Petroff, F., Etienne, P., Creuzet, G., Friederch, A. and Chazelas, J., “Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices”, Phys. Rev. Lett., 61 (1988) 2472), or tunnel magnetoresistance (Moodera, J S., Kinder, L R., Wong, T M. and Meservey, R., “Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions”, Phys. Rev. Lett 74, (1995) 3273-6).
Furthermore, it has also been observed that, by passing a spin-polarized current through a thin magnetic film, reversal of its magnetization can be induced in the absence of any external magnetic field (Katine, J. A., Albert, F. J., Buhrman, R. A., Myers, E. B., and Ralph, D. C., “Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars”, Phys. Rev. Lett. 84, 3149 (2000).).
The polarized current can also generate sustained magnetic excitations, also called oscillations (Kiselev, S. I., Sankey, J. C., Krivorotov, L N., Emley, N. C., Schoelkopf, R. J., Buhrman, R. A., and Ralph, D. C., “Microwave oscillations of a nanomagnet driven by a spin-polarized current”; Nature, 425, 380 (2003)). Using the effect of generating sustained magnetic excitations in a magnetoresistive device makes it possible to convert this effect into an electrical resistance modulation which can be used directly in electronic circuits, and can therefore directly affect the frequency. Document U.S. Pat. No. 5,695,864 describes various elements employing the physical principle mentioned above. It describes, in particular, the precession of the magnetization of a magnetic layer through which a spin-polarized electrical current passes. The physical principles involved, as well as the terminology used, are also described and defined in Patent Application FR 2 892 271.
The oscillation frequency of these radiofrequency oscillators is varied by adjusting the strength of the current passing through them, and optionally also an external magnetic field.
Patent Application US 2007/0 259 209 describes a radiofrequency oscillator comprising:                a free layer, the magnetization of which precesses when a spin-polarized current passes through it perpendicularly to its plane,        an injector of spin-polarized current into the free layer in order to make its magnetization precess in a sustained fashion, this injector having a spin-polarized current injection face directly in contact with the free layer,        a magnetoresistive contact having a measurement face directly in contact with the free layer in order to form, in combination with the free layer, a tunnel junction making it possible to measure the precession of the magnetization of the free layer,        a conductive pad directly in contact with the free layer in order to make an electrical current flow through the injector without passing through the magnetoresistive contact.        
The conductive pad is particularly advantageous. This is because without it all of the current passing through the injector would also have to pass through the tunnel junction. The resistivity of the tunnel junction would then need to be low enough to be able to withstand the high current densities necessary for precession without breakdown. Tunnel junctions of low resistivity would need to have a very thin barrier (for example less than one nanometer).
The tunnel barrier is a layer of nonmagnetic material making it possible to magnetically isolate a reference layer of the magnetoresistive contact from the free layer while preserving the spin polarization of the current which passes through it.
Nonmagnetic materials are defined here as materials not having a measurable magnetization in the absence of an external magnetic field. They may for example be materials with no magnetic property, or paramagnetic materials or diamagnetic materials.
A very thin tunnel barrier corresponds to a very low RA factor. The RA factor is the product of the resistance of the tunnel barrier and its area (here, the area is the surface area of the cross section of the tunnel barrier).
When the RA factor of the tunnel barrier is lower, the variation range of the resistivity of the tunnel junction is commensurately smaller (for example less than 10%) and the sensitivity of the tunnel junction to the precession of the magnetization in the free layer is commensurately less.
It will therefore be understood that in the absence of this conductive pad, an apparently insoluble physical contradiction would exist since the tunnel barrier needs to be both thin and thick, which makes it necessary to resort to unsatisfactory compromises.